In a four-cycle engine, the blow-by gas which leaks through the gap between the piston and the cylinder collects in the crank chamber, and there is a possibility that the pressure in the crank chamber is abnormally raised. Therefore, it is necessary to provide a breather hole to permit the increased pressure to escape to the outside. However, since oil is located in the crank chamber and is violently stirred therein, the crank chamber is thus filled with scattered fine oil particles, so that there is an inconvenience such that if a simple breather hole is provided, then the oil component is also simultaneously drained to the outside through this breather hole. To avoid this, there is used a breather apparatus in which the exhaust gas is stored in a breather chamber (expansion chamber) having a proper volume, and the oil component is separated by a reduction in pressure and flow rate and is collected.
FIG. 1 shows an example of a known breather apparatus which is used in the engine for use in two-wheeled motorcycles. Breather chambers c and d are formed in the rear and rear upper portions in a main casing b which includes therein a crank chamber a. The gas passes via through holes e and f and is transferred through the breather chambers c and d and the oil component is separated. Thereafter, the gas is exhausted from a breather hole g into an air cleaner casing or the like. The separated oil is transferred through an oil return passage h, which is especially provided, and is returned to the oil in the oil pan.
In such a breather apparatus, in particular, since the oil return path h is arranged in the rear portion of the main casing b because of limitations on the available space, the casing becomes long due to the size of the mission casing. Therefore, the frame increases in size and the attaching position to the frame is located forwardly and the center of gravity is shifted to the front position, so that an adverse influence is exerted on safety in the running of the two-wheeled motorcycle.
On the other hand, in some engines of the overhead cam type, a breather chamber is formed in a head cover by use of a constitution such that the crank chamber is communicated with the inside of the head cover through a cam chain chamber. However, there are several problems in that the head portion of the engine increases in size and, in order to avoid interference with the fuel tank, the attaching position of the engine is lowered so that the road clearance decreases, and the like.
As mentioned above, most of the conventional breather apparatuses for four-cycle engines are arranged in a part of the inside of the main casing or head cover, so that there are problems in that the existence of the breather chamber causes the sizes of the mission casing and head cover to increase, and the degree of freedom in design and layout in installing the engine is reduced.
Also, in four-cycle engines, an operational valve cam to control the opening and closure of intake and exhaust valves is driven by a cam chain in accordance with the timing of the rotation of the crank shaft. In overhead cam type engines, in general, a cam chain is suspended in a cam chain chamber formed so as to vertically penetrate the cylinder block and cylinder head. Since the timing of the cam chain is disrupted when the tension is released, a chain tensioner is provided for one side of the cam chain, thereby giving a proper tension thereto. An adjuster apparatus is used to keep the proper tension.
The adjuster apparatus elastically presses the chain tensioner by the force of a spring. In general, the adjuster apparatus has a backward motion-preventing mechanism such as a ratchet mechanism or the like for inhibiting the backward motion in the releasing direction of the chain. However, in this case, it is necessary to provide a further mechanism to release the backward motion-preventing mechanism in order to permit the cam chain to be detached for maintenance or the like.
Therefore, in order to enable the backward motion-preventing mechanism to be easily released, the adjuster apparatus is ordinarily arranged outside the cylinder block. In this case, the projecting portion on the outside of the cylinder block is enlarged, so that the outside appearance becomes bad and the other parts are obstructed by this projection. Although there is a constitution such that the adjuster apparatus is disposed in the cylinder block by providing a space large enough to easily release the backward motion-preventing mechanism, this creates problems in that the size, weight, and cost of the engine increase.
As mentioned above, in the conventional cam chain tensioner adjuster apparatus for four-cycle engines, either the adjuster apparatus is disposed so as to project to the outside of the cylinder block or it is attached in the cylinder block by forming it with extra space therein Thus, there are problems such that the adjuster apparatus obstructs the easiness of layout and makes the external view bad, and the engine size increases and the like.